The term “windmill” today refers to a modern wind turbine, a complex machine designed to convert the kinetic energy of moving air into electrical power. The power a single turbine generates is not fixed, but rather a spectrum determined by its physical design and location. Output ranges from small amounts suitable for an individual home to massive capacities for utility grids. Understanding the actual delivered power requires looking beyond the turbine’s maximum rating to consider constantly changing atmospheric conditions.
The Core Determinants of Power Output
The potential power output of any wind turbine is governed by a fundamental physical relationship involving three primary variables: wind speed, rotor size, and air density.
Wind speed is the most influential factor because available power is proportional to the cube of the wind velocity, a relationship known as the cubic law. Doubling the wind speed results in an eightfold increase in the theoretical power available for capture, making site selection with predictable, high winds paramount for economic viability.
The second major determinant is the size of the rotor, specifically the area it sweeps. Power is directly proportional to the swept area, meaning that doubling the length of the blades quadruples the potential energy capture. This drives the industry trend toward massive blades. The physical limits of energy conversion are defined by the Betz limit, which states that no turbine can convert more than 59.3% of the wind’s kinetic energy into mechanical energy.
A third factor is the density of the air, which decreases with increasing altitude and temperature. Taller towers are constructed to elevate the rotor into faster, less turbulent winds, reducing the effect of ground friction and obstacles. The average hub height for newly installed onshore turbines in the U.S. reached over 103 meters in 2023.
Power Output by Turbine Scale
Wind turbines are broadly categorized by their maximum rated electrical output, referred to as nameplate capacity, which varies across residential, community, and utility scales.
Small-scale or residential turbines are typically rated between 1 kilowatt (kW) and 10 kW, designed to supplement or power a single home or small farm. A mid-range 5 kW turbine, operating in favorable conditions, can generate approximately 8,000 to 9,000 kilowatt-hours (kWh) of electricity annually.
Medium-scale or community turbines have nameplate capacities ranging from 100 kW to 500 kW. These machines are utilized for small businesses, schools, or local grid support in rural areas.
Utility-scale turbines form the backbone of modern wind farms, representing the largest class, with capacities generally starting at 2 megawatts (MW). The average onshore utility turbine installed in the U.S. in 2023 was rated at 3.4 MW. A single 2-3 MW capacity turbine can generate around 6 million kWh of electricity each year, enough to power approximately 1,500 average households.
Offshore utility turbines are consistently the largest models, benefiting from stronger and more consistent sea winds. Prototypes are reaching 16 MW or more in nameplate capacity. A single 16 MW offshore turbine can generate up to 80,000 megawatt-hours (MWh) annually, sufficient to power over 20,000 homes. Increasing turbine size allows developers to generate more power with fewer machines, lowering the overall cost of energy production.
Understanding Power Metrics and Consistency
Nameplate capacity, measured in kilowatts (kW) or megawatts (MW), represents the maximum instantaneous electrical power the generator can produce under optimal wind conditions. Since the wind does not blow steadily at this optimal speed, a turbine rarely operates at its full rated capacity. This distinction highlights the difference between power (the rate of energy flow) and energy (the total output delivered over time).
To quantify the actual energy delivered, the industry uses the capacity factor. This is the ratio of the actual energy produced over a period to the maximum possible energy it could have produced at full nameplate capacity. Expressed as a percentage, this metric gives a realistic measure of a turbine’s real-world performance. Typical capacity factors range from 20% to 40%, depending heavily on the site’s wind resource.
Onshore wind farms typically operate with capacity factors averaging around 33.5%. Offshore wind farms generally achieve higher factors, often between 29% and 52%, because winds over open water are stronger and less intermittent than on land. While a turbine may be rated at 5 MW, a 40% capacity factor means its average output is equivalent to running continuously at 2 MW (40% of 5 MW).